Frankfurt to Rome

I recommend to fly-in to Berlin and after that travel by train to Nuremberg and Munich. From there Berchtesgaden, Salzburg ongoing to Rome. Please ask Austria and Italian forum on route recommendtions in these countries. Then you will cover two major citites and really many sights of WWII in Germany which is an insufficient "naming" to what people sort under it. By the way: What do you sort under it? Places of war actions, war memorials, Nazi terror regime, Holocaust, ...?

Nürnberg and Munich are indeed the obvious places in this part of Germany for your husband's WW2 interests. (Frankfurt Airport has an hourly direct train to Nürnberg.)
Munich has a direct train connection to Northern Italy, crossing the Dolomites mountain range via the Brenner Pass. You could break your journey in Innsbruck.
In Italy you have a wide range of options for the route to Rome. Venice and Florence are obvious choices, either staying in these cities themselves or nearby (such as Padova for Venice and Lucca for Florence).

On the website of the trustworthy Man in Seat 61 you'll find all practical information on European trains, including the repeatedly asked questions whether a rail pass is a good choice (mostly not) and whether 1st class is recommended (2nd class is comfortable enough).
The train planner of the Deutsche Bahn is a perfect tool for looking up rail connections. Tickets for trains outside Germany must be bought with the relevant national company, such as Trenitalia or Italo for Italy, or with their agent Trainline.

This history buff finds Berlin soooo much more interesting than Munich. I've spent almost 30 days in Berlin the last 3 years vs 3 days in Munich. I'm headed back to Berlin for another 15 or so days this year. Munich? I feel like i saw all that I wanted to see in 3 days. So, I agree with MarkK... start/land in Berlin if you can. Gee whiz... just landing in Berlin takes you to a historic place... Tegel Airport, whose original runway was built in 1948 to support the Berlin Airlift (the current commercial terminal complex was built in the 60's).

Does the two week time frame include your two flight days? Also, where are you flying from?

Given the very short two week time frame, my suggestion would be to start in Munich. That's where the Nazi party started and there are a lot of historic sites in that area. A few thoughts.....

• Flight inbound Munich - take S-Bahn in from the airport - check RS Germany guidebook for hotel suggestions.
• While in Munich, you could take Third Reich tours with Radius Tours, including to Dachau (or you can go on your own). You could also travel by train or bus to Berchtesgaden and tour the Dokumentation Centre, Eagle's Nest or other sites.
• travel by train from Munich to Florence. On the current schedules, there's a departure at 09:34, arriving 17:15 (time 7H:41M, one change at Bologna Centrale). That's a full day of travel, but the trip through the Brenner Pass is always very scenic.
• While in Florence you might enjoy touring some of the popular sights there. The RS Italy book has lots of good information about hotels, the various museums and how to minimize queues. If you're interested, you could also pay a visit to the American Cemetery near Florence.
• Take the train to Rome (90 minute trip, very easy at 300 km/h).
• While in Rome, you can tour whichever of the usual sites you're interested in. If you want to see a few more WW-II sites, you could also take a day trip to Anzio (about one hour each way by train). There's a Museum of the Landings there as well.
• Flight outbound from Rome.

There are a few caveats to be aware of when using trains and other public transit in Italy. If you need more information, post another note.

I'm not sure you'll have time for a short cruise.

Good luck with your planning.

Tegel Airport, whose original runway was built in 1948 to support the
Berlin Airlift

And the airport was up and running in 90 days (in the first version) ... BER in Brandenburg is version 0.95 since years.

I appreciate all of your thoughts and recommendations. Now I can start my planning.